This invention relates to nuclear reactors and primarily to fuel sub-assemblies for liquid metal cooled nuclear reactors.
A fuel assembly for a liquid metal cooled nuclear reactor may comprise a multiplicity of closely spaced, parallel fuel pins between which liquid metal coolant can be flowed. To facilitate replacement of fuel pins, in one known construction of liquid metal cooled nuclear reactor, the pins are arranged to form fuel sub-assemblies each comprising a bundle of pins enclosed within a tubular shroud or wrapper and having means for plugging the lower end into a diagrid member. Coolant is flowed upwardly from the diagrid through the wrappers of the sub-assemblies. The fuel sub-assemblies are usually arranged in groups, the pins of some groups being more enriched in fissile isotope than those of other groups and varied flows of coolant are required to optimise the coolant outlet temperature of the sub-assemblies. Whilst varied flows can be effected by gagging the inlet ports of each sub-assembly on assembly of the core, difficulties arise owing to the change in heat flux generated in the sub-assemblies due to varying rates of burn-up and batch replacement of sub-assemblies. Removal of fuel sub-assemblies from the fuel assembly containing vault merely to effect a change of gagging is a complex operation and therefore means is desired whereby variations in gagging can be effected whilst the fuel sub-assemblies are housed within the vault.